Doesn't crash nix-build on spago build errors

[chekoopa]

Got some errors on missing main module from Parcel, and only few retries later my eye caught few compie errors. After quick research I've realized that buildSpagoStyle script always exits with zero code, which would be inconvenient and error-prone.

I propose replacing:

purs compile ...
echo done.

with something like

purs compile ... && echo done.

So it would notify at successful builds only and keep build flow consistent.

[justinwoo]

the related code is here: https://github.com/justinwoo/spago2nix/blob/master/src/Generate.purs#L148-L168

do you want to make a PR for setting -e flags for the scripts? we can also remove them altogether to be honest.